1. Field of the Invention
This invention relates to thermal ink jet printing devices and, more particularly, to thermal ink jet printheads having a channel geometry which controls the location of the bubble collapse on the heating elements, so that the cavitational forces do not directly impact the heating element/electrode interfaces.
2. Description of the Prior Art
Though thermal ink jet printing may be either a continuous stream type or a drop-on-demand type of ink jet printing, its most common use is that of drop-on-demand. As a drop-on-demand type device, it uses thermal energy to produce a vapor bubble in an ink-filled channel to expel a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle and, specifically, a predetermined distance upstream therefrom. The resistors are individually addressed with an electrical pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
The environment of the heating element during the droplet ejection operation consists of high temperatures, frequency related thermal stress, a large electrical field, and a significant cavitational stress. The mechanical stress, produced by the collapsing vapor bubble, in the passivation layer over the heating elements are severe enough to result in stress fracture and, in conjunction with ionic inks, erosion/corrosion attack of the passivation material. The cumulative damage and materials removal of the passivation layer and heating elements result in hot spot formation and heater failure.
Upon further investigation, it has been found that the bulk of all heating element failures occur not on the resistor which vaporizes the ink, but rather at the junction or interface between the resistor and the addressing electrode connection the resistor to its driver.
The ink jet industry has recognized that the operating lifetime of the ink jet printhead is directly to the number of cycles or bubbles generated and collapsed that the heating element can endure before failure. Various printhead design approaches and heating element constructions are disclosed in the following patents to mitigate the vulnerability of the heating elements to cavitational stress, but none have controlled the location of the bubble collapse on the heating element to prevent it from collapsing near an electrode connection by channel geometry.
U.S. Pat. No. Re. 32,572 to Hawkins et al, discloses several fabricating processes for ink jet printheads, each printhead being composed of two parts aligned and bonded together. Many printheads can be simultaneously made by producing a plurality of sets of heating element arrays with their addressing electrodes on, for example, a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channels and associated manifolds are produced in a second silicon wafer and, in one embodiment alignment, openings are etched thereon at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks and then bonded together and diced into many separate printheads.
U.S. Pat. No. 4,638,337 to Torpey et al discloses an improved thermal ink jet printhead similar to that of Hawkins et al, but has each of its heating elements located in a recess. Recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blow-out, which causes ingestion of air and interrupts the printhead operation whenever this event occurs. In this patent, a thick film organic structure, such as Riston.RTM., is interposed between the heater plate and the channel plate. The purpose of this layer is to have recesses formed therein directly above each heating element to contain the bubbles generated by the heating element, enabling an increase in droplet velocity without the occurrence of vapor blow-out.
U.S. Pat. No. 4,774,530 to Hawkins discloses an improved printhead which comprises an upper and lower substrate that are mated and bonded together with a thick insulative layer sandwiched therebetween. One surface of the upper substrate has etched therein one or more grooves and a recess, which when mated with the lower substrate, will serve as capillary filled ink channels and ink supplying manifold, respectively. Recesses are patterned in the thick layer to expose the heating elements to the ink, thus placing them in a pit and to provide a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the groove closed ends to the manifold recess so that the printhead fabrication process is simplified.
U.S. Pat. No. 4,835,553 to Torpey et al discloses an ink jet printhead comprising upper and lower substrates that are mated and bonded together with a thick film insulative layer sandwiched therebetween. One surface of the upper substrate has etched therein one or more grooves and a recess which when mated with the lower substrate will serve as capillary filled ink channels and ink supply manifold, respectively. The grooves are open at one end and closed at the other. The open ends serve as nozzles. The manifold recess is adjacent the grooved closed ends. Each channel has a heating elements located upstream of the nozzle. The heating elements are selectively addressable by input signals representing digitized data signals to produce ink vapor bubbles. The growth and collapse of the bubbles expel ink droplets from the nozzles and propel them to a recording medium. A recess patterned in the thick layer provides a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels and increase the flow area to the heating elements. Thus, the heating elements lie at the distal end of the recesses so that a vertical wall of elongated recess prevents air ingestion while it increases the ink channel flow area and increases refill time, resulting in an increase in bubble generation rate.
U.S. Ser. No. 07/330,574 filed March 30, 1989 to Hawkins, entitled "Thermal Ink Jet Device with improved Heating Elements", now U.S. Pat. No. 4,935,752, discloses a thermal ink jet printhead which uses heating element structures which space the portion of the heating element structures subjected to the cavitational forces produced by the generation and collapsing of the droplet expelling bubbles from the upstream aluminum electrode interconnection to the heating element. In one embodiment this is accomplished by narrowing the resistive area where the momentary vapor bubbles are to be produced, so that a lower temperature section is located between the bubble generating region and the electrode connecting point. In another embodiment, the electrode is attached to the bubble generating resistive layer through a doped polysilicon descender. A third embodiment spaces the bubble generating portion of the heating element from the upstream electrode interface, which is most susceptible to cavitational damage, by using a resistive layer having two different resistivities.
U.S. Pat. No. 4,897,674, to Hirasawa, discloses a thermal ink jet printhead having a plurality of nozzles, an ink reservoir, and a plurality of parallel ink channels, with heating elements therein which provide ink flow paths from the reservoir to the nozzles. The cross-sectional area of the channels gradually decreases from the reservoir to the nozzles. Small walls are provided on the side of the channel adjacent the reservoir for the purpose of diminishing the loss of energy applied to the ink which escapes toward the reservoir.
The U.S. Pat. No. 4,638,337 improved the Reissue U.S. Pat. No. Re. 32,572 by providing an intermediate thick film layer between the heating element substrate and the channel wafer. The thick film layer is etched to expose the heating elements, thus placing them in a pit whose walls prevent lateral movement of the droplet emitting bubbles and prevent vapor blow-out and the ingestion of air that causes printhead failure. The U.S. Pat. No. 4,774,530 simplified the fabrication of the printheads by adding the etching of an ink flow path in the thick film layer between the reservoir and the channels. The ink channel cross-sectional flow areas prevented rapid refill with ink during the printing operation, slowing the printing speed. The U.S. Pat. No. 4,835,553 corrected this by creating a larger etched recess in the thick film layer by enlarging the thick film etched recess to connect and combine the heating element recess or pit and the ink flow passageway between the channels and the reservoir. Thus, the two basic types of thermal ink jet printheads are the separate or full pit structure of U.S. Pat. Nos. 4,638,337 and 4,774,530, schematically shown in FIGS. 2A and 2B, and the open pit structure of U.S. Pat. No. 4,835,553, schematically shown in FIGS. 3A and 3B. These prior art schematics are discussed in more detail later.
In U.S. Pat. No. 4,935,752, the problem of the collapsing bubble damaging the electrode interface with heating element was recognized as the reason most heating element failures occurred, and it solved this problem by designing the heating element so that the bubble generating region was always spaced from the upstream electrode interface.
The prior art printheads basically fall into three types of structures: the full pit structures, represented by FIGS. 2A and 2B; the open pit structures, represented by FIGS. 3A and 3B; and the no pit structures disclosed in U.S. Pat. Nos. Re. 32,572 and 4,935,752, to Hawkins. Experimental data shows that the bubble collapse of the no pit and a full pit configurations is near the upstream end of the heating element and the heating element failure takes place because of damage at the address electrode interface. High velocity fluid impact, referred to as cavitational stress or damage, appears to be the cause of this damage, and numerical modeling studies corroborate this behavior. Numerical modeling studies have shown that the bubble collapse for the open pit geometry takes place near the front, or downstream end, of the heating element, subjecting the common lead connection to cavitational damage, and experimental data have confirmed this.